Providing several interconnection levels is highly desirable in very large scale integrated circuit (VLSI) technology, as it allows both higher packing density of the active components and greater freedom in their placement on the chip. An interconnection layer typically includes an insulating layer on which a conductive interconnection pattern is formed or in which it is embedded. An interconnection layer is typically formed of two conductor-insulator layers, including an upper layer providing horizontal paths among areas of the chip and a lower conductor-insulator layer providing vertical connections between the conductors in the upper layer and the interconnection layer or the semiconductor therebeneath. The connection in the lower conductor-insulator layers of each interconnetion layer ar called solid vias which ar through the insulating layer and filled with conductive material provide connections to the underlying interconnection layers or semiconductor devices.
Care must be taken in fabricating the chip to ensure that each interconnection layer is provided with a relatively planar upper surface before the next interconnection layer is applied. Unless this is done, sharp corners and discontinuities, known as steps, can develop in the surface of an interconnection layer. If metal is being used as the conductive material, which is preferred to polysilicon because of its lower resistivity, problems can develop in the interconnections because metals typically have poor coverage over corners and steps. Accordingly, as each interconnection layer is applied, it is desirable to ensure that the top surface of each interconnection layer has a relatively planar upper surface before beginning the next layer.
One approach to achieving a planar upper surface has been the use of a reflowed glass technique. In this technique an interconect layer is covered by a glass, which is then heated close to its melting point so that it "reflows", leaving a planar upper surface. The glass, usually silicon dioxide, has a quite high melting point. The melting point of the silicon dioxide can be reduced by the introduction of phosphorus into the silicon dioxide, which reduces the glass's melting point to about 1000 degrees Celsius. The melting point of the glass can be reduced by about another 100 degrees by the addition of boron to the glass. The drawbacks of the reflowed glass technique stem from the high temperatures required to melt the glass, which temperatures may be higher than the melting points of the metals used as conductors. For example, the reflowed glass technique cannot be used to smooth a surface above an interconnection layer in which aluminum is the conductor, because of aluminumn's relatively low melting point. There is also a risk of metal corrosion with the formation of phosphoric acid over time. In addition, boron can diffuse out of the silicon dioxide and into the underlying silicon, which can result in instability in the threshold voltage of MOS transistors.
Refractory metals can be used in place of aluminum as the interconnect material in the reflowed glass process. The high melting points of refractory metals ensure that they will not melt during the reflow process. A problem with using such metals, however, is that at temperatures over about 700 degrees Celsius, metal silicide may form where the metal comes in contact with the silicon substrate to form connections, which can increase the resistance of the connection.
Another approach to achieving a planar interconnection layer is the use of lift-off techniques. A layer of resist is used as a mask in the etching of recesses in the surface of the chip. A layer of metal is then formed having thickness comparable to the depth of the recesses, thereby filling the recesses with metal. The metal outside of the recesses is removed by dissolving underlying the layer of resist, which causes the metal on top of the resist to lift off, leaving the metal in the recesses.
A disadvantage of the use of lift-off is the requirement that metal be deposited at low temperatures (about 150 degress Celsius) in order to avoid decomposition of the layer of resist on which the metal is deposited. Metal deposited at such low temperatures is subject to recrystallization and hillock formation during further processing steps of the integrated circuit chip.